This invention relates to microelectronic devices, and, more particularly, to the structure and use of a metal-oxide-semiconductor field effect transistor having low noise at temperatures below about 40K.
Microelectronic devices are electrical components and circuits of very small size that are fabricated in a generally planar fashion in a semiconductor substrate. Circuit elements are fabricated on the substrate, and are joined to other circuit elements by external leads. Such devices are widely used in civilian and military applications because of their small size, low weight, and high speed of operation.
One application of microelectronic devices is processing the outputs of sensors such as an infrared ("IR") sensor array. The infrared sensor array contains sensor elements that are responsive to energy radiated in the infrared range. The output of the sensor elements must be amplified and processed, eventually leading to an infrared image of the field of view of the sensor array. There are typically hundreds to hundreds of thousands of sensor elements in the sensor array, each of whose outputs is individually processed and then combined in the final image.
The infrared sensor typically operates at a very low, cryogenic temperature of less than 20K. ("K" refers to degrees Kelvin, which is measured from absolute zero.) This low sensor temperature increases the sensitivity of the sensor to infrared radiation and allows weaker infrared sources to be measured or detected. It is desirable to conduct at least a portion of the electronic processing of the output of the sensor elements in a readout circuit at a location immediately adjacent to the sensor elements, and typically in an electronic package that is integral with the sensor array.
The electronic amplifying and processing is circuitry therefore constructed of microelectronic devices that also operate at the very low temperatures at which the sensor operates. At the present time, the only suitable semiconductor device technology that can operate at such low temperatures is the metal-oxide-semiconductor field effect transistor (widely known as a MOSFET). Conventional MOSFETs introduce an undesirable deionization noise component to the signal at these low temperatures. The noise component limits the signal-to-noise ratio of the readout signal. This deionization noise has been observed to be particularly troublesome for MOSFETs operated at less than 40K.
In the past, the problem of the inherent noise introduced by the MOSFET has been observed. However, its mechanism has not been well understood, and there has been no approach to removing the noise. There is therefore a need for an improved approach to reducing the inherent input referred noise of MOSFET-type microelectronic devices that must be operated at temperatures of less than about 40K. The present invention fulfills this need, and further provides related advantages.